sexy formal dresses

Hello fellow Whovians, I have the Hot Topic tardis dress and im trying to think of ways to spice it up for a formal ball at Collosalcon East next month. I was planning to wear a black petticoat under it. I also own tardis earrings and a necklace. And i do have a mini black top hat with a tardis bow attached I can wear with it but im not sure its formal enough? Suggestions? sexy formal dresses